Evidence for Li+/H+ Exchange during Ambient Storage of Ni-Rich Cathode Active Materials

Layered Ni-rich transition metal oxides like so-called NCMs are one of the most-promising high-energy density cathode active materials (CAMs) for next-generation Li-Ion batteries. However, compared to NCMs with low nickel content, Ni-rich NCMs suffer from a highly reactive surface that leads to an accumulation of surface contaminants and also from a higher soluble base content. Since a detailed understanding of the formation rate of surface contaminants is still lacking, we will investigate the effect of extended storage in high relative humidity air of a Ni-rich NCM851005 (Li1+delta (Ni0.85Co0.10Mn0.05)(1-delta) O-2, with delta typically similar to 0.005-0.03) and a low nickel content NCM111 (Li1+delta (Ni1/3Co1/3Mn1/3)(1-delta) O-2) on the build-up of surface contaminants. The formation rate of the surface contaminants during this accelerated wet-storage test is quantified by TGA-MS under Ar. To elucidate the processes occurring during the TGA-MS experiments, as-received and wet-stored CAMs are introduced into an XPS chamber where they are heated in situ to different temperatures, followed by XPS analysis of the surface compositional changes. Comparative measurements with water-washed NCM851005 reveal the close analogy between the processes that occur during extended storage of NCMs in humid ambient air and during the washing of NCMs, commonly used to lower the soluble base content of Ni-rich NCMs.